How to make a tape measure for exact measurements?
First, two assumptions. One is that I assume your person model is made in compliance with the usual norms as far as topology and geometry of the model. The other is that the "exact measurements" to which you refer to in the title of your question relates to the exact measurements made and communicated to you, and not necessarily to obtaining exact measurements in the model. That written, here's what I would do. 


Make a duplicate of your person model and move it to a different layer. 
Figure out where on the model you will need to show the measurements, likely including (but not limited to) the neck, the chest, the upper and lower arms, the waist, the hips, and upper and lower legs. 
At each point where you have identified a location where you need a measurement, define a contrasting material to any that might already be present on your real person model, and make two loop cuts around the duplicate person model in that location. Space those two loop cuts you just made to be about an inch apart, or so (relative to the model), and assign the new material to the areas between the two loopcuts. 
When all of the areas where you need measurements have been marked, in the material context button of the object properties window, in edit mode, choose "select" to select all of the loop cuts with the material you need, then in the select menu in the 3D viewport header, choose "inverse selection". Now delete everything from the duplicate model that is not a location of a measurement you need.
Switch the pivot point to "median point", and scale each of the locations of the tape up by about 2 percent, constraining the scale directions so as not to change the scale of the tape in the direction perpendicular to the tape. Do not change the locations of any of your measurement lines. 
Move the tape lines from the layer you moved them to in step one, back to the layer where the person model is to be found. 


If you choose, before doing step 6, you can make a more accurate and detailed representation of the ends of your tape measure